The field of the invention is the correction of nearsightedness (myopia), farsightedness (hyperopia), and astigmatism by changing the shape of the cornea.
Refractive corneal surgery which alters the shape of the cornea can be performed using a number of techniques. Lamellar refractive keratoplasty involves the placement of material in, or on the cornea to change its shape. For example, a disk of cornea material from a donor is sutured to the anterior surface of the cornea, or a synthetic material is implanted within the corneal stroma. Keratotomy changes the shape of the cornea by making a pattern of incisions in the cornea's anterior surface. Keratectomy involves the removal of a piece of the cornea in order to change the contour of its anterior surface, and thermokeratoplasty involves the application of heat. The heat is applied in a pattern which brings about the desired change in corneal shape and the heat may be produced by a thermal probe, a laser, or a radio frequency current.
All of the above techniques for altering the shape of the cornea, except for the last one, thermokeratoplasty, involves physical penetration of the anterior layers of the cornea. A well known way of practicing keratotomy, for example, involves the making of a series of incisions in the cornea in a radial pattern at a depth ranging from 80% to 95% of the cornea thickness. These incisions may be made with cutting instruments or with lasers, and they create lines of weakness around the periphery of the cornea which cause slight peripheral bulging and consequent central flattening of the cornea. This slight central flattening decreases the focusing power of the optic system to provide the desired correction for myopia. Keratotomy has the disadvantages associated with surgical incisions into the cornea, including risk of infection, risk of perforation of the cornea, loss of the eye, double vision and glare. In addition, there is difficulty in accurately controlling the amount of correction in the focusing power.
Keratectomy also involves cutting into the cornea and removing or ablating sections thereof to alter its shape. For example, an excimer laser may be used to sculpt the central region of the cornea to alter its curvature. This sculpting process involves the application of laser energy to the anterior surface of the cornea which ablates the cornea to the desired depth. To correct for 6 diopters of myopia, for example, an approximately 50 micron thickness must be removed from the anterior surface of the cornea in the corneal center. The major difficulty with this procedure is the removal of the anterior layers of the cornea, including the epithelium, basal membrane and Bowman's membrane. This can result in infection, recurrent erosion of the epithelium, and scarring of the corneal stroma and replacement of some of the ablated tissue with collagen that is less regular and less clear.
Thermokeratoplasty seeks to deliver heat to the underlying corneal stroma and it has been recognized heretofore that when heat is applied to the corneal stroma the shape, or contour, of the cornea will change. However, the problem has been to deliver a proper amount of thermal energy to the corneal stroma without damaging the anterior surface of the cornea and without damaging the delicate endothelium, the most posterior layer of the cornea. While efforts have been made and proposals have been made to use lasers for thermokeratoplasty, none have been particularly successful and none have been tried to correct myopia. Instead, the preferred techniques at the present time include the use of an electrically heated probe which is inserted through the anterior surface of the cornea and into the corneal stroma (i.e. penetrating thermokeratoplasty), or the application of radio frequency probes to the surface of the cornea which induces heating in the cornea.